The present invention relates to a heat transfer cover film. More particularly, the present invention relates to a heat transfer cover film enabling heat transferred images to be improved in terms of such durability as rub resistance and allowing them to develop color and luster so well. The present invention also concerns a heat transfer process making use of such cover films.
So far, heat transfer techniques have been widely used for simple and expeditious printing. Allowing various images to be produced expeditiously, these heat transfer techniques have incidentally been employed for prints usually made in a small number, e.g. for preparing ID or other cards.
Where it is desired to obtain color images like photographs of face, another type of heat transfer technique is now available, making use of heat transfer films of continuous length comprising a continuous substrate film on which a number of heat transfer layers colored in yellow, magenta and cyan (and black, if necessary) are formed successively and repeatedly.
Such heat transfer sheets are generally broken down into two types, one referred to as a so-called wax type of heat transfer film in which a heat transfer layer is thermally softened and transferred onto an image-receiving material in an imagewise manner and the other a so-called sublimation type of heat transfer film in which only a dye sublimes (migrates) thermally from within a heat transfer layer onto an image receiving sheet after an imagewise pattern.
When ID or other cards are to be produced with such heat transfer films as mentioned above, the wax type of heat transfer film has the advantage of being capable of forming verbal, numerical or other images, but involves the disadvantage that such images are poor in durability, esp., rub resistance.
With the sublimation type of heat transfer film, on the other hand, it is possible to obtain gray scale images, i.e., gradation pattern, like photographs of face. Unlike those obtained with ordinary ink, however, the formed images are less lustrous for lack of any vehicle and, by the same token, are poor in durability, e.g. rub resistance.
In order to solve such problems, it has been proposed so far to laminate transparent films on the surfaces of the images. However, this is not only cumbersome to handle but gives rise to card curling as well, because the cards are laminated all over the surfaces. What is more, too thin films cannot be used in view of lamination work, thus posing a problem that the overall thickness of cards increase.
As an alternative to the above-mentioned lamination technique, it has been proposed to coat the surfaces of images with heat- or ionizing radiation-curable resins and cure them. However, this is not only troublesome to handle but also brings about a possibility that the images may be attacked by solvents in coating materials. With the heat-curable resins, there is another possibility that the dyed images may discolor or fade due to the heat used for curing.
It is therefore an object of this invention to provide a heat transfer cover film which can solve the above-mentioned problems of the prior art and so can expeditiously give excellent, curl-free images that are improved in terms of such properties as durability, esp. rub resistance, luster, color development. Another object is to provide a heat transfer process making use of such a cover film.
The above-mentioned and other objects and features of the invention are achievable by the following aspects of the invention.
The first aspect of this invention concerns a heat transfer cover film characterized in that an ionizing radiation-cured resin layer is releasably formed on a substrate film.
By forming an ionizing radiation-cured resin layer on a substrate film in a releasable manner and transferring that layer onto the surface of a transfer image, it is possible to provide expeditious production of an excellent, curl-free image representation which is improved in terms of such properties as durability, esp. rub resistance, gloss and color development.
In a particularly preferable embodiment, a relatively large amount of transparent particles may be incorporated in the ionizing radiation-cured resin layer, whereby a protective layer having a much more improved rub resistance is heat transferable, because the film can be well cut during heat transfer.
The second aspect of this invention concerns a heat transfer cover film characterized in that a wax-containing transparent resin layer is releasably formed on a substrate film.
By forming a wax-containing resin layer on a substrate film in a releasable manner and transferring it onto the surface of a transfer image, it is possible to provide expeditious production of an excellent, curl-free image representation which is improved in terms of such properties as durability, esp. rub resistance, gloss and color development, since that layer can be easily transferred onto the image by the heat heat used for printing.
The third aspect of this invention concerns a heat transfer cover film characterized in that a silicone-modified transparent resin layer is releasably formed on a substrate film.
By forming a silicone-modified transparent resin layer on a substrate film in a releasable manner and transferring it onto the surface of a transfer image, it is possible to provide expeditious production of an image representation which is improved in terms of such properties as durability, esp. rub resistance, chemical resistance and solvent resistance, since the transparent resin layer is easily transferable onto the image by the heat used for printing.
The fourth aspect of this invention concerns a heat transfer cover film including a substrate film having a transparent resin layer releasably formed thereon, said resin layer being further provided on its surface with a heat-sensitive adhesive layer, characterized in that said heat-sensitive adhesive layer is made of a resin having a glass transition temperature or Tg lying between 40xc2x0 C. and 75xc2x0 C.
By constructing from a resin with a Tg of 40-75xc2x0 C. a heat-sensitive adhesive layer provided on the surface of a transparent resin layer, the transparent resin layer can be well transferred onto an image through a thermal head while it is kept in good xe2x80x9cfoil cuttingxe2x80x9d condition. Thus the transparent resin layer is so easily transferred on the image by the heat of the thermal head that an image representation improved in terms of such properties as durability, esp. rub resistance, chemical resistance and solvent resistance can be obtained expeditiously.
The fifth aspect of this invention concerns a heat transfer process in which (a) a dye layer of a heat transfer sheet including a substrate film having said dye layer on its surface is overlaid on (b) a dye-receiving layer of a heat transfer image-receiving sheet including a substrate film having said dye-receiving layer on its surface in opposite relation; heat is applied from the back surface of said heat transfer sheet according to an imagewise pattern to form an image; and a transparent protective film is laminated on the surface of said image, characterized in that said dye layer contains a releasant, while said dye-receiving layer is releasant-free or contains a releasant in such an amount as to offer no impediment to the lamination of said transparent protective layer.
By allowing the dye layer to contain the releasant in an amount sufficient to ensure easy release of it from the dye-receiving layer during heat-transfer while permitting the dye-receiving layer to be releasant-free or contain the releasant in such an amount as to offer no impediment to the lamination of the transparent protective layer, it is possible to laminate the transparent protective layer easily on the surface of the image formed by heat transfer and thereby produce an image representation which is improved in terms of such properties as durability, esp. rub resistance, resistance to staining, light fastness, resistance to discoloration and fading in the dark and storability.
It is a further object of this invention to provide a heat transfer sheet enabling an image having an improved gray scale to be easily produced simultaneously with high-density verbal, numerical or other images. This object is achievable by the following aspect of the invention.
The sixth aspect of this invention concerns a heat transfer sheet in which a substrate sheet is provided on the same surface with a first heat transfer layer comprising a thermally migratable dye and an untransferable binder and a second heat transfer layer comprising a dyed or pigmented, heat-meltable binder, characterized in that said substrate sheet is made of a polyester film treated on at least its surface to be provided with said heat transfer layers in such a way that said surface is made easily bondable.
By using as a substrate sheet a polyester film made readily bondable to heat transfer layers, it is possible to provide a heat transfer sheet enabling a clear grayscale image and a clear verbal or other image to be made at the same time.
Such a heat transfer sheet as described above is especially useful for forming the images required to have a cover film. For that purpose, this heat transfer sheet may also have a transparent layer for such a cover film as mentioned just above.